The disclosures of Japanese Patent Application No. 2000-106957 filed on Apr. 7, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a structure for joining thermoplastic resin parts to each other.
2. Description of Related Art
In a fuel supply system of an automobile, making various parts out of plastic has been proposed for weight reduction. Since within a fuel supply system a fuel tank is a relatively large part, the use of plastic for this part largely contributes to weight reduction of the vehicle. Thus, in recent years, many different plastic fuel tanks have been implemented.
When a plastic fuel tank is used, it is difficult to mount different members such as a fuel cut valve and onboard refueling vapor recovery (ORVR) valve to be connected to the fuel tank. When joining plastic parts, fusing is normally employed, but joining by fusing can be difficult depending on the type of resin, making it difficult to improve airtightness or prevent the member from coming off.
For example, Japanese Patent Application Laid-Open Publication No. HEI 10-71861 discloses related art which is a mounting structure for a fuel cutoff valve to a plastic fuel tank. The mounting structure of the related art is described hereafter with reference to FIG. 6.
A skin layer of a fuel tank 1 is composed of high-density polyethylene (HDPE) from the viewpoint of strength and durability. On the other hand, a fuel cutoff valve 2 is composed of polyacetal (POM). The difference in materials employed for the fuel tank 1 and the fuel cutoff valve 2 leads to problems in fusing due to the difference in melting points of the respective materials. Consequently, joining the members stably for an extended period of time is difficult.
To solve this problem, in this structure, the fuel cutoff valve 2 is joined to a high-density polyethylene (HDPE) fuel cutoff valve mounting member 3 by insert molding (joining) first, and then the fuel cutoff valve mounting member 3 in which the fuel cutoff valve 2 is insert-molded is joined to the fuel tank 1.
As another example, Japanese Patent No. 2715870 discloses another related art which is a pipe joining structure which is described with reference to FIG. 7.
A pipe 11 on one side is composed of high-density polyethylene (HDPE) corresponding to the material of a tank. On the other hand, a pipe 12 on another side is composed of polyamide (PA). This polyamide resin does not exhibit a good joining property with high-density polyethylene so it is difficult to fuse these materials directly.
Therefore, the pipe 11 on the one side composed of high-density polyethylene (HDPE) is insert molded (joined) with the portion 13 of the pipe 12 on the other side.
According to the aforementioned related arts for a joining structure for plastic parts, however, there have been problems such as the following.
First, a resin such as high-density polyethylene (HDPE) does not exhibit an excellent joining property with other resins such as polyamide (PA), polybutylene terephthalate (PBT), polyacetal (POM), polyethylene terephthalate (PET), or polyphenylene sulfide (PPS) so it is difficult to maintain a joined state and airtightness over an extended period of time.
Therefore, it is an object of the invention to provide a joining structure in which the joining structure of plastic parts is improved so that the joined state is maintained as well as airtightness for an extended period of time.
In order to achieve the aforementioned object, a joining structure as a first aspect of the present invention includes a first plastic part composed of at least one thermoplastic resin of polyamide (PA), polybutylene terephthalate (PBT), polyacetal (POM), polyethylene terephthalate (PET), or polyphenylene sulfide (PPS), a second plastic part composed of at least high-density polyethylene (HDPE), and a metamorphic polyethylene layer that comes in contact with both the first plastic part and the second plastic part for joining the first plastic part and the second plastic part.
The joining property between such a resin as high-density polyethylene (HDPE) and other resins including polyamide (PA), polybutylene terephthalate (PBT), polyacetal (POM), polyethylene terephthalate (PET), or polyphenylene sulfide (PPS) is not particularly good. However, a joined state and airtightness can be maintained stably for an extended period of time by joining the two members with a metamorphic polyethylene that exhibits an excellent joining property with both of the members.